Diffraction gratings are frequently used in lasers for reflecting back into a laser""s resonating cavity only a narrow range of wavelengths of light centered at one particular wavelength. Light energy at this wavelength resonates within the cavity and is emitted through a partially reflective mirror at the other end of the cavity. Examples of such diffraction gratings and various methods of making these diffraction gratings are described in U.S. Pat. Nos. 5,080,465; 5,436,764; and 5,493,393, incorporated herein by reference.
Typically, a master diffraction grating is first manufactured. This master grating is then used to form many replica gratings. Each of these replica gratings may then be used as a master grating for forming other replica gratings.
As described in the ""465 patent, a master grating may be formed by depositing aluminum over a substrate, such as glass. A diamond tool under interferometric control may then be used to rule very closely spaced grooves in the aluminum layer. The separation of the grooves is related to the wavelength of the light to be reflected by the grating and to the narrowness of the range of wavelengths it is required to reflect. In one embodiment, the diamond tool rules on the order of tens of thousands of grooves per inch. The diffraction grating surface may be ten square inches and the grating one inch thick. Creating a precision master grating by physical ruling is, therefore, an extremely time consuming and expensive process.
Once a master grating has been made, replicas of the grating may be made using techniques such as are described in an article by Torbin and Wiskin in Soviet Journal of Optical Technology, Vol. 40(3) (March, 1973): 192-196. In one such method, a release agent, such as silver, gold, copper glycerine, carnuba wax, debutyphthalate or low vapor pressure oil is coated on the surface of the master. A thin (e.g., 1 micron) reflective layer, such as aluminum, is then deposited onto the release layer. An uncured polyester cement (epoxy) may then be deposited on the aluminum layers and a glass or metal substrate is then placed on top of the epoxy. After the cement is cured, the glass layer, epoxy layer, and aluminum layer are then separated from the master grating, resulting in a replica of the master grating.
Magnesium fluoride is a known optical coating. Coatings of this material having thicknesses of xcex/4 are used to reduce unwanted reflections. Also MgF2 coatings have been shown to improve the efficiency of gratings operating at wavelengths greater than about 500 to 600 nm. (See Maystre, et. al, Applied Optics, Vol. 19(18) (Sep. 15, 1980): 3099-3102. Al2O3 and SiO2 are also well known coating materials.
One important use of replicated gratings is to line narrow excimer lasers producing ultraviolet light at wavelengths of 248 nm and 193 nm. Applicant has discovered that prior art replica gratings suffer substantial performance degradation when subject to intense ultraviolet radiation especially at the higher energy 193 nm wavelength. What is needed are replica gratings capable of long term high quality performance in intense ultraviolet radiation.
The present invention provides an overcoat protected diffraction grating. A replica grating having a thin aluminum reflective grating surface is produced by replication of a master grating or a submaster grating. The thin aluminum reflective surface may be cracked or have relatively thick grain boundaries containing oxides and hydroxides of aluminum and typically is also naturally coated with an aluminum oxide film. The grating is subsequently recoated in a vacuum chamber with a thin, pure, dense aluminum overcoat and then also in the vacuum the aluminum overcoat is coated with a thin protective layer. In a preferred embodiment this protective layer is a single layer of MgF2. In another preferred embodiment the layer is a layer of MgF2 covered with a layer of Al2O3 and in a third preferred embodiment the protective layer is made up of four alternating layers of MgF2 and Al2O3. The thin protective layer not only protects the aluminum from ultraviolet caused degradation but also improves the normal reflectivity of the reflecting faces of the grating. The grating is especially suited for use for wavelength selection in an ArF laser operating producing an ultraviolet laser beam at a wavelength of about 193 nm. The oxygen free aluminum overcoat prevents the ultraviolet light from causing damage by stimulating chemical reactions in grating materials under the aluminum grating surface or in the aluminum oxide film.